The present invention relates to highly carboxylated cellulose fibers and a process for making such fibers. The highly carboxylated cellulose fibers of the inventions are water-insoluble and have enhanced absorbency toward water and body fluids, making them desirable for use in personal hygiene articles, high strength paper making, cellulose ester coatings with low volatile organic components (VOC), as well as in many other applications. The process of the invention is based on the reaction of organic dicarboxylic acid anhydrides, such as phthalic anhydride, maleic anhydride, succinic anhydride, glutaric anhydride, trimellitic anhydride, 1,2-cyclohexanedicarboxylic anhydride, and oxalyl chloride and cellulosic fibers.
Carboxylated cellulose fibers have been used and proposed for use in a number of applications where the presence of carboxyl groups on the fibers is believed to enhance some properties of the cellulose fibers. However, the limited extent to which cellulose fibers could heretofore be carboxylated in a cost-effective and environmentally benign fashion has limited the use of such fibers.
Polymer composites or blends employing cellulose exhibit limited compatibility with certain polymeric materials, including nylon-6 and polypropylene. This incompatibility diminishes the mechanical properties of the polymer composite or blend products. The compatibility of cellulose to such polymeric materials is improved by adding carboxylic acid groups to the cellulose, where it is believed that the carboxyl groups enhance the compatibility between polymers and cellulose.
A need also exists for water-insoluble fibers having improved absorbency towards water or body fluids such as urine, blood, mucus, menses, lymph and other body exudates. Fibers having improved absorbency would find ready application in areas such as personal hygiene, medicine, house keeping, clothing and electronics, as well as in other products. One of the most important applications of water-insoluble fibers having improved absorbency is in disposable absorbent articles, such as diapers or incontinence pads. It would be particularly desirable if articles incorporating such fibers could be processed using conventional commercial equipment. To enable such processing, improved absorbency fibers must meet certain minimal values with regard to fiber strength and fiber length.
In the art of papermaking, there are materials which are used to improve the wet strength of paper. These materials are known in the art as xe2x80x9cwet strength agents.xe2x80x9d Cationic wet strength agents are perhaps the most widely used variety. The effectiveness of cationic wet strength agents is often limited by the low retention of the wet strength agent on conventional cellulose fibers. This low retention is frequently due to the cationic agents not finding suitable anionic sites for attachment to the fiber, which causes them to remain in solution or to be washed off the fiber after application. Although cationic promoters can be used to increase wetting agent retention, they do not increase the number of anionic sites on a fiber surface, and in some cases may actually decrease the number of such sites, thus inhibiting the wet strength agent from performing its function. It is desirable to increase the number of anionic sites on a fiber to improve the efficiency of wet strength agents. The anionic sites on conventional cellulose pulps can be measured in terms of the carboxyl group content of cellulose, which is typically in the range of about 20 to about 120 milliequivalents per kilogram (meq/kg) of cellulose. U.S. Pat. No. 5,935,383 discloses a method for improving the efficiency of aqueous cationic wet strength additives by pretreating cellulose surfaces with reactive anionic compounds, thus providing the cellulose surface with additional anionic sites suitable for retaining cationic wet strength additives on the cellulose.
Cellulose esters are often used in pharmaceuticals and industrial coatings. However, they frequently exhibit relatively low solid contents in suitable solvents, necessitating use of large amounts of solvent. The use of high solvent levels is undesirable since it is associated with prolonged drying times and atmospheric contamination through solvent evaporation. Although solvent borne cellulose esters provide desirable coatings properties, the current trend is to formulations which require reduced amounts of the volatile organic components (VOC), or which employ water soluble coating formulations, thereby entirely eliminating VOC. This trend has limited the use of solvent borne cellulose esters in coatings applications. WO99/40120 describes an attempt to make carboxylated cellulose esters having improved solvent solubility to enable high solids coating compositions. The method described in WO 99/40120 utilizes oxidized cellulose which is activated with water. The water in the activated cellulose is then displaced with acetic acid and the product esterified and then hydrolyzed. Cellulose esters using carboxylated cellulose fibers as starting material prepared according to the present invention overcome the poor solubility of conventional cellulose esters in aqueous media and thus reduce the need to use VOC in the production of coatings, for example, cellulose esters.
Cellulose fibers having high carboxyl content would be useful in all of the above applications. Carboxylated cellulose can be made through: (a) oxidation of cellulose, (b) etherification of cellulose with monochloroacetic acid, (c) esterification of cellulose with some dicarboxylic acid anhydrides or chlorides, such as phthalic anhydride, maleic anhydride, succinic anhydride and oxalyl chloride.
Some oxidants such as hypohalite, chlorine dioxide, nitrogen dioxide (dinitrogen tetraoxide), permanganate, dichromate-sulfuric acid and hypochlorous acid can be used to make carboxylated cellulose fiber; however, the obtained oxidized celluloses (or oxycelluloses) either have low carboxyl content (lower than 250 meq/kg) or very low intrinsic viscosity as measured in cupriethylenediamine (Cuene I.V.) In addition, some oxidized celluloses may contain aldehyde and/or ketone functionalities besides carboxyl group depending on the nature of the oxidant and the reaction conditions used in their preparation. This can impair their performance as coatings. Sodium (or potassium) periodate is a very effective oxidant, however, it cannot be used cost effectively because there is no viable method to recover periodate. Furthermore, carboxylated cellulose fibers made by the periodate method with carboxyl contents higher than 1000 meq/kg, have Cuene I.V. less than 2 dL/g, which limits their applications.
Etherification of cellulose by monochloroacetic acid yields carboxylated cellulose (carboxymethyl cellulose) with relatively high carboxyl content and high I.V.; however, the carboxylated cellulose products produced in this fashion are usually particles, instead of fibers, if the degree of substitution (DS) of carboxyl group is higher than 0.3. Particles are susceptible to water absorption except when present in the acid form and/or crosslinked. U.S. Pat. No. 4,410,694 discloses a method for preparing carboxylated fibers in water using monochloroacetic acid; however, the degree of substitution (DS) of the carboxylated fibers is very low.
U.S. Pat. No. 4,734,239 describes the production of water-insoluble fibers of cellulose monoesters of maleic acid, succinic acid and phthalic acid, having an high absorbability for water and physiological liquids. The carboxylated cellulose was prepared via esterification of cellulose by dicarboxylic acid in the presence of dimethylacetamide/lithium chloride (DMAc/LiCL) as solvent and potassium acetate as catalyst. The solvent medium, DMAc/LiCL is costly to use and DMAc is toxic. Further, the carboxylated cellulose produced according to the patent is substantially dissolved and must be spun to produce a fiber.
Accordingly, there exists a need for an economical and environmentally benign method of making highly carboxylated fibers from polysaccharide fibers, including wood cellulose, which retain their fiber form during carboxylation and which have sufficient fiber strength to be processed into commercial articles, and in particular absorbent articles, utilizing conventional processing equipment.
There is also a need for biodegradable disposable articles for personal hygiene, medical and domestic use. The carboxylated cellulose fibers of the invention are suitable for use in such articles. Highly carboxylated cellulose fibers can replace, partially or totally, base fiber non-woven materials in wipes and disposable articles, and fiber/super absorbent polymer mixes in absorbent products, to make biodegradable disposable absorbent articles.
The present invention provides water-insoluble, highly carboxylated cellulose fibers which are suitable for use in cellulose acetate coatings, absorbent core materials, high wet strength papers and polymer composites. The highly carboxylated cellulose fibers of the invention retain their fiber form throughout the carboxylation process and have sufficient fiber strength and length to be processed using conventional fiber processing technology. They can be made with a wide range of intrinsic viscosities and a high degree of substitution (xe2x80x9cDSxe2x80x9d) of carboxyl groups.
The highly carboxylated water-insoluble cellulose fibers of present invention are made by reacting cellulose with dicarboxylic acid anhydrides or chlorides using weak organic acid, and a base or basic salt. Preferably the weak organic acid acts as both a dispersing agent and induces fiber swelling while the base or basic salt acts as a catalyst. The fibers produced possess a unique combination of high carboxyl content and high intrinsic viscosity. The range of carboxyl content which can be achieved using the invention is from about 150 to greater than 4000 milliequivalents per kilogram (meg/kg). The range of viscosities achievable with the highly carboxylated cellulose fibers according to the invention is from about 0.5 dl/g to about 12 dl/g. This combination of carboxyl content and viscosity enable the carboxylated cellulose fibers of the invention to be utilized in a wide variety of applications, including absorbent products, health care products, specialty papers, adhesives, detergents, biodegradable fibers, ion exchange fibers and as precursors for aqueous coatings.
The highly carboxylated cellulose fibers of the invention can be made with any fibrous polysaccharide material, including cellulosic pulp derived from softwood pulp, such as various pines (Southern pine, White pine, Caribbean pine), Western hemlock, various spruces (e.g., Sitka Spruce), Douglas fir, from hardwood pulp sources, such as gum, maple, oak, eucalyptus, poplar, beech, or aspen, and from cotton linters, bagasse, cereal straw, reeds, kenaf, bamboo, and regenerated fibers such as rayon and lyocell, and mixtures of all of the foregoing. The cellulose fibers useful in the invention can be subjected to mechanical and/or chemical pretreatment, such as defiberization, bleaching, mercerization or chemical modification, prior to carboxylation.
In accordance with the invention a suitable cellulose fiber having an average Cuene I.V. in the range of from about 2 to about 15, and preferably in the range of from about 3 to about 13, is:
(a) dispersed in an weak organic acid to form a suspension having a consistency of about 0.5% to about 20% by weight, and preferably about 1% to about 15% by weight, at a temperature from about 15xc2x0 C. to about 60xc2x0 C., and preferably at about 20xc2x0 C. to about 50xc2x0 C.;
(b) the obtained suspension of cellulose and weak organic acid is reacted with a dicarboxylic acid anhydride or an anhydrous dicarboxylic acid chloride in a mole ratio from about 0.1:1.0 to 10:1.0 (dicarboxylic acid anhydride or chloride : cellulose), and preferably in a mole ratio of about 0.5:1.0 to about 5:1.0, at about 50xc2x0 C. to about 118xc2x0 C., and preferably from about 60xc2x0 C. to about 100xc2x0 C., in the presence of a basic catalyst, over a period from about 0.3 hours to about 15.0 hours, and preferably from about 0.5 hours to about 12.0 hours, and most preferably from about 2.0 hours to about 8.0 hours, to obtain carboxylated cellulose fibers; and
(c) the produced carboxylated cellulose fibers are separated from the reaction suspension by filtering, or centrifuging, or other separation method.
Where necessary or helpful, such as for fibers intended for absorbent material application, the carboxylated cellulose fibers of the invention can be partially or completely converted into the corresponding fiber-shaped salts by direct reaction with alkali metal hydroxide, carbonate, acetate, alkali metal alcoholates, ammonia or primary or secondary amine.
It is essential for obtaining carboxylated fibers with satisfactory mechanical characteristics that the fibers have sufficiently high degree of polymerization. It is therefore essential that the starting polysaccharide fibers display an average Cuene I.V. from 3.0 to 13.0, preferably from 4.0 to 13.0, which should be substantially maintained upon the reaction with dicarboxylic acid anhydrides or chlorides. The starting materials can be wood cellulose pulps including hard wood pulp and soft wood pulp, non-wood pulp such as cotton linter, bagasse or cereal straw or regenerate fiber such as Rayon and lyocell. The starting material can be also other polysaccharides such as starch, chitin, chitosan or pullulan.
The esterifying reagent can be a dicarboxylic acid anhydride such as phthalic anhydride, maleic anhydride, poly (maleic anhydride), succinic anhydride, glutaric anhydride or a dicarboxylic acid chloride, such as oxalyl chloride. The esterifying reagent can also be 1,2,4-benzenetricarboxylic anhydride, 1,2-cyclohexanedicarbocylic anhydride or mixtures of all of the foregoing. The esterifying reagents useful in the invention have the below general structures: 
wherein R1, R2, R3, R4=H, alkyl, aryl, halogen, carboxyl, carboxyalkyoxyl or amide; and 
wherein R=alkyl or aryl; X=halogen, xe2x80x94CN or CONH2; and mxe2x89xa70, nxe2x89xa71, oxe2x89xa70, pxe2x89xa70 and qxe2x89xa70.
Generally, the esterifying reagents are employed in amounts from 10 to 1000% by weight relative to the starting cellulose fiber, depending on the carboxyl content of product needed. To avoid the hydrolysis of esterifying reagents and the degradation of cellulose, a weak acid dispersant, such as acetic acid or other organic acid, must be used. The reaction temperature and reaction periods must be adjusted relative to each other. Reaction temperatures of from about 50xc2x0 C. to 130xc2x0 C., and preferably about 60xc2x0 C. to about 118xc2x0 C., with reaction times of about 0.3 hours to about 15 hours, and preferably 0.5 hours to about 12 hours, are believed to yield carboxylated cellulose esters of the invention having desirable properties. Reaction temperatures from 70xc2x0 C. to 118xc2x0 C. and reaction times from 2 to 5 hours have proven to be particularly advantageous for the reaction of the invention.
Various acids are well known catalysts for the esterification reaction. Unexpectedly these acids are not suitable for the reaction of cellulose and dicarboxylic acid anhydride, as they not only result in fibers having low intrinsic viscosities and/or products not having a fiber shape, but also because of the low carboxyl content of the products made with them. However, basic esterification catalysts are well suitable for cellulose esterification reactions of the invention, especially suitable are those catalysts that minimize the degradation of the cellulose. By way of example, the following tertiary amines are useful in the present invention 4-N,N-dimethylaminopyridine, collidin, pyridine and triethylamine. Preferred as esterification catalysts are basic salts of monocarboxylic acids, such as sodium acetate, potassium acetate, sodium propionate, potassium propionate, sodium butyrate and potassium butyrate. Generally, these basic salts are employed in amounts from about 0 (i.e., not present) to about 150% by weight, and preferably from about 5% to about 50% by weight, and most preferably from about 10% to about 20% by weight, relative to the cellulose fibers treated.
The obtained carboxylated cellulose fibers can be characterized by solid state C13 NMR, Cuene I.V. measurement, and carboxyl content measurement using a conductometric titration method.
A diaper incorporating the carboxylated fiber according to the invention comprises: (a) a liquid impervious backing sheet; (b) a relatively hydrophobic, liquid pervious topsheet; (c) a flexible absorbent core positioned between said backing sheet and the topsheet. The flexible absorbent core comprises of hydrophilic fiber material and optionally particles of a substantially water-insoluble hydrogel material, known as a super absorbent polymer (SAP). The highly carboxylated polysaccharide ester fibers of the invention can replace, partially or totally, the super absorbent polymer conventionally used in many absorbent articles.
The carboxylated cellulose fibers of the invention can also be used in feminine hygiene articles and other articles wherein absorbent fibers find application. The structure and method of fabrication for such articles are well known to those skilled in the art of the invention.
The advantage of the present invention is that it does not require toxic solvents and does not require spinning technology to produce a fibrous material.